This invention relates to the production, from two different liquid input streams which are at different temperatures, of a single output stream with a selectable, low-average-thermal-noise regulated temperature which is intermediate the input streams' temperatures. The invention, while having clear applicability in a number of different settings, is described hereinbelow in connection with a film-processing system in which it has been found to offer particular utility.
There are many fields, such as the field of photographic processing, where regulated control of the temperature of a liquid, such as water, is critical. For example, and in the field specifically of film processing, an inadvertent change of even a fraction of a degree in the temperature of water which is used during processing can affect seriously the quality of the final processed image. Often, this kind of processing takes place substantially automatically in various types of film-processing machines. One such type of machine, wherein the method and apparatus of the present invention have been found to offer a decided advantage, is a machine like that described in U.S. Pat. No. 3,695,162--the disclosure of which is incorporated herein by reference, simply for the purpose of setting the background of the invention. Machines like this have been available for a number of years, often fitted with mechanical-type temperature-control sub-systems. However, such known sub-systems have been shown to experience considerable difficulty in maintaining a proper preselected water temperature, where input variables, such as flow-rate, pressure, and hot and cold water temperatures, fluctuate, as is customary in the usual public water-supply system.
A general object of the present invention is to provide a unique method and apparatus which results in merger, into a single, low-average-thermal-noise, temperature-regulated stream, water flows received from the usual hot and cold water outlets in a facility.
More particularly, an object of the invention is to provide such a method and apparatus in which the usual to-be-expected changes in flow-rate, pressure and temperature in such hot and cold water streams are quickly taken care of, so that in the merged outlet stream, very precise temperature control is achieved, with extremely low-average-level thermal noise.
According to a preferred embodiment of, and way of practicing, the present invention, hot and cold inlet water streams, derived from the usual water supply in a facility, are fed intermittently, and in a pulsed fashion, into an output conduit which directly feeds a heat-exchanging coil formed of copper, or of another suitable material having a relatively high thermal conductivity. A fixed-duration, recurrent operating cycle is defined, during each one of which cold water flows for a predetermined first part of the interval, and hot water flows during the remainder of the interval. Temperature sensors are placed on the discharge side of the output conduit and on the discharge end of the coil. These sensors are coupled to a control circuit which compares the responses produced by the two sensors with an adjustable bias voltage whose level is settable, substantially infinitely throughout a defined range, to establish the desired regulated temperature in the final output stream. The control circuit controls actuation of valving apparatus, which in turn determines when hot, and when cold, water flows. The fixed interval referred to above remains unchanged, and temperature control is achieved by adjusting the percentages of that interval characterized by hot and cold water flows. The particular flow periods for each of the two input streams during a given interval are referred to as the duty cycles for the two streams.
For example, if, during a given interval, cold water flows for 30% of the time, and hot water flows for 70% of the time, the resulting blend will exhibit a temperature which is warmer than if the reverse situation were the case. Regardless of pressures, flow-rates or temperatures of the input streams, so long as the regulated temperature which is called for is between the temperature extremes of the input streams, very precise temperature control, with extremely low average thermal noise, is achievable. In fact, tests conducted using the invention have shown its capability to maintain the average temperature of a merged output stream to within about .+-.0.2.degree. F. with respect to the regulated temperature called for.
These and other objects and advantages which are achieved by the invention will become more fully apparent as the description which now follows is read in conjunction with the accompanying drawings.